iOS 14

Go to your App Library (last screen on the right on your home screens), hold down on the app and select "Add to Home Screen"

iOS 12-13

If you have hidden apps from your home screen and you want to get them back, simply restart your phone (Hold power button) and all apps should return to your home screen.

If you want your apps to appear in Siri and Search, go to Settings > <app name> >and enable Siri and Search